The present invention relates to apparatus for use in dilating occluded blood vessels. The invention is particularly concerned with such apparatus wherein dilatation is achieved through the means of a balloon element which is initially inverted within the distal end of a catheter and, in use, extruded through and expanded within the occlusion being treated. The invention is additionally concerned with such apparatus having double lumen characteristics for the catheter to enable the taking of pressure measurements or the making of injections while the catheter remains within the vessel being treated.